One method for the preparation of caprolactam involves the liquid-phase catalytic hydrogenation of phenol to form cyclohexanone, the reaction of formed cyclohexanone with hydroxylamine sulfate to produce cyclohexanone oxime, and Beckmann rearrangement of the formed cyclohexanone oxime with oleum to form crude caprolactam according to Kirk-Othmer Encyclopedia of Chemical Technology 4, 830-832 (1992).
Typical impurities in such crude caprolactam are benzene, cyclohexanone oxime, aniline, octahydrophenazine, acetic acid, phenol, adipimide, ammonium benzenesulfonate, ammonium 3-cyclohexanone sulfonate, and ammonium 2-hydroxycyclohexanone-sulfate. Methods for purifying such crude caprolactam include oxidation as taught by U.S. Pat. Nos. 4,148,792; 4,178,287; 4,248,781; 4,314,940; 4,720,328; 5,350,847; and 5,637,700; extraction as taught by U.S. Pat. Nos. 4,148,793; 4,170,592; 4,301,073; and 4,606,858; ion exchange as taught by U.S. Pat. Nos. 5,245,029 and 5,440,032; hydrogenation as taught by U.S. Pat. Nos. 5,032,684; 5,502,184; and 5,539,106; crystallization as taught by U.S. Pat. Nos. 4,493,719; 4,795,571; 4,882,430; 4,900,821; and commonly assigned U.S. Pat. No. 2,813,858; and distillation as taught by U.S. Pat. Nos. 4,326,925; 4,328,154; 4,457,807; 4,610,768; 5,441,607; 5,458,740; and commonly assigned U.S. Pat. No. 4,767,503.
Another method for the preparation of caprolactam involves depolymerization of nylon production scrap as taught by U.S. Pat. Nos. 3,939,153; 4,605,762; 5,233,037; 5,241,066; 5,359,062; and 5,495,014. U.S. Pat. No. 5,458,740 teaches that the wastewater from the polymerization of caprolactam contains polycaprolactam which may be depolymerized and the resulting caprolactam may then be purified by adding inorganic acid such as sulfuric acid to caprolactam and water mixture and distilling at 666-1066 Pa and between 165-180.degree. C. However, the use of sulfuric acid is disadvantageous because the sulfuric acid may catalyze the formation of aminocaproic acid and oligomers.
Recently, methods have been developed for the depolymerization of waste nylon-containing carpet as taught by commonly assigned U.S. Pat. Nos. 5,457,197; 5,681,952; 5,869,654; and 5,929,234. U.S. Pat. No. 5,681,952 teaches a process for depolymerizing multi-component waste material comprising polycaprolactam and non-polycaprolactam components to form caprolactam. The process comprises the step of: in the absence of added catalyst, contacting the multi-component waste material with superheated steam at a temperature of about 250.degree. C. to about 400.degree. C. and at a pressure within the range of about 1 atm to about 100 atm and substantially less than the saturated vapor pressure of water at the temperature wherein a caprolactam-containing vapor stream is formed. The reference teaches that caprolactam may be separated from other components of the distillate by sending the vapors from the reactor overhead to a partial condenser to obtain a condensate containing caprolactam. The reference also teaches that fiber grade caprolactam may be obtained from this condensate by further purification including distillation, crystallization, and other conventional techniques known in the art and that for example, the caprolactam purification process of AlliedSignal's U.S. Pat. Nos. 2,813,858; 3,406,176 or 4,767,503 to Crescentini et al. may be used.
U.S. Pat. No. 5,681,952 teaches that the crude caprolactam was submitted to fractional distillation under vacuum and a fraction containing over 99% caprolactam was obtained and that less than 10% of the available caprolactam remained in the distillation bottoms. The distilled caprolactam was further purified via crystallization from water to yield fiber quality caprolactam.
Carpets include a face fiber that is adhered to a support material such as jute or polypropylene backing, latex (such as a styrene-butadiene rubber (SBR)), and a variety of inorganic materials such as calcium carbonate, clay, or hydrated alumina fillers. Nylon 6 is often used for the face fiber. Typically, carpet comprises about 20-55 percent by weight face fiber and 45-80 percent by weight backing materials. In addition, the fiber contains dyes, soil repellents, stabilizers, and other compounds added during fiber and/or carpet manufacture. Waste carpet may also contain a host of other impurities, which will collectively be referred to herein as "dirt". Decomposition products including 6-aminohexanoic acid; caprolactam dimer; N-methylcaprolactam; N-ethylcaprolactam; hexenoic acid; cyclohexylamine; hexamethylene diamine (HMDA); and acetic acid; and non-nylon-6 derived components such as 1,3-diphenylpropane; styrene dimer; styrene-butadiene oligomers; and acids, aliphatic alcohols such as 1-decanol and 1-dodecanol and carboxylic acids with 6 to 16 carbon atoms per molecule contaminate the depolymerized polycaprolactam and need to be removed in order to obtain world class caprolactam. The term "world class caprolactam" as used herein means caprolactam having a purity greater than 99.9 weight percent (excluding water)and a permanganate number less than 3 and a color number less than 2.
U.S. Pat. No. 5,169,870 and International Publication WO94/06763 teach the depolymerization of nylon-6 carpet in the presence of phosphoric acid and steam to form crude caprolactam which is first purified by condensing and fractionating volatile components from the crude caprolactam. Potassium permanganate is then added to the resulting crude caprolactam and water mixture wherein the permanganate oxidizes the impurities which are not removed in the fractionation step. This method is disadvantageous according to U.S. Pat. Nos. 5,556,890 and 5,637,700 because solid manganese dioxide is produced during the purification which then has to be removed from the reaction by means of filtration. See also U.S. Pat. Nos. 5,455,346; 5,495,01 5; and 5,536,831. U.S. Pat. No. 5,556,890 teaches that depolymerized nylon-6 carpet may be purified by hydrogenation in the presence of a hydrogenation catalyst. Hydrogenation is a disadvantageous process because it requires a capital intensive step and rather than lower the total impurities, hydrogenation only changes the impurities to another substance.
A need in the art exists for a process for purifying caprolactam obtained from the depolymerization of polyamide-containing carpet which avoids the preceding problems in the art. We depolymerized polyamide-containing carpet and then attempted to purify it by crystallization alone. Unfortunately, crystallization produced poor quality crystals which could not be washed adequately and the purity of the resulting caprolactam was unsuitable for commercial use as reported in Comparative Example 1 below. We also depolymerized waste nylon-containing carpet and then attempted to purify it by distillation alone. The results were unacceptable as reported in Comparative Example 2 below. Example 8 of U.S. Pat. No. 5,681,952 does not teach or suggest a distillation temperature, distillation pressure, the crystallization conditions, or the final caprolactam purity.